A computing system having a database creates an index to speed up processing of stored data, and also updates an index at the time of deletion and update of stored data and addition to stored data. An index which supports the entire index target data stored in the database is referred to as a complete index. On the other hand, an index whose data structure is not incomplete and which supports only part of the index target data is referred to as an incomplete index.
In relation to a computing system which processes a query from a client by using an index, it is proposed as a first related art relating to the present invention to select one of the following two processing methods when an index corresponding to a query is incomplete (see Patent Document 1, for example):
(1) abnormally end the query; and
(2) suspend processing of the query, execute an index creation process so that the index becomes complete, and then restart the suspended processing of the query.
    Patent Document 1: Japanese Unexamined Patent Application Publication No. H5-094475    Non-Patent Document 1: Junichi Tatemura, Oliver Po, Wang-Pin Hsiung, and Hakan Hacigumus. 2012. Partiqle: an elastic SQL engine over key-value stores. In Proceedings of the 2012 ACM SIGMOD International Conference on Management of Data    Non-Patent Document 2: Pouria Pirzadeh, Junichi Tatemura, Hakan Hacigumus, “Performance Evaluation of Range Queries in Key Value Stores,” 2012 IEEE 26th International Parallel and Distributed Processing Symposium Workshops & PhD Forum, pp. 1092-1101
In the case of using an incomplete index, it is difficult to obtain the same result as in the case of using a complete index, because an incomplete index supports a less amount of data. However, because the data structure of an incomplete index is not incomplete, processing does not break down even if an incomplete index is used for a query. Therefore, it may be more advantageous for a client in some cases to process a query immediately by using an incomplete index than abnormally end a query without exception or keep the client waiting long until an incomplete index becomes a complete index. However, the first related art relating to the present invention does not present any idea about processing a query by using an incomplete index as it is.